The consistence of the spleen is not very great : its parenchyma is soft and doughy, readily yielding to the pressure of the finger. It is not unfrequently torn by mechanical injury during life ; indeed, more easily than any other glandular organ, especially if it be over-distended with blood at the time; but, under the opposite circumstances, it is much less disposed to give way. The colour of the spleen is bluish red, during life greyish violet, and the parenchyma is of a dark dusky red.
Structure. — In the spleen we first distin guish the coverings or involucra, and the paren chyma or proper spleen-substance. The first consists of the serous and the fibrous mem brane. The latter is composed of a frame work of reticulated fibres firmly connected together, constituting the so-called trabecular tissue (trabeculw lienis) ; and, beside this, of the red spleen-substance, the splenic cor puscles, and vessels and nerves, together with sheaths which arise from the fibrous coat.
1. The serous nzembrane (tunica serosa) is a part of the peritoneum. It accurately covers the outer surface of the spleen as a tnnooth membrane, with the exception of its hilus only, where it takes the form of two folds which convey the vessels of the organ, con stituting the gastro-splenic ligament, and pass ing off to the stomach, where they become continuous with its serous covering. When the ligament uniting the spleen to the dia phragm exists, the membrane is similarly con tinuous with the peritoneum covering this muscle. The serous membrane is a thin, mo derately strong, whitish membrane, which is intimately connected with the fibrous coat ; although in particular places, and especially after previous maceration, the two may be separated from each other. In respect of its microscopical•structure, it scarcely differs at all from other parts of the visceral layer of the peritoneum; thus it consists of an outer and single layer of polygonal pavement epithelium, and of an inner layer of white fibrous tissue, in which meshes of fine fibre of yellow tissue are present in no very considerable quantity.
In mammalia, e. g. in the sheep, ox, &c., as was remarked by Malpighi, the serous membrane is easily separated entire. But in man this is not the case, and hence Haller and others have supposed that only one membrane is present. But microscopical re search proves the opinion to be erroneous ; and pathological anatomy confirms this state ment, by showing that the outer part of the coat of the spleen shares in the diseases of the peritoneum. In animals numerous vessels
are seen in the serous membrane, and a very dense network of stronger and thicker fibres of yellow tissue is present.
2. The fibrous coat (tunica fibrosa, ginea, sive propria) is in man a moderately delicate semi-transparent, but firm, membrane, which encloses the parenchyma of the spleen on every side, so as to include it in a kind of sac. Its outer surface is even, and in man is intimately united with the serous covering, with the single exception of the hilus, where the two membranes diverge, and are separated from each other by vessels, nerves, and a loose areolar tissue. The inner surface bounds the parenchytna of the organ, and, with the exception of very nutnerous solid processes which come off from it, is limited by the becular tissue. At the hilus of the spleen it sinks into the interior of the organ in the shape of tubes (vagithE vasorunz), which sheath the entering and emerg,ing vessels, and are continued on these throughout the whole parenchyma. The fibrous coat, in the human subject, is composed of white fibrous tissue, mixed with elastic or yellow fibres. The former of these, as in other fibrous membranes, consists of bands, which take a parallel course, but do not form distinct bundles ; and the latter are united in a very dense and irregular network. Duvernoy and Stukely have scribed muscular fibres in this tunic ; but, cording to my searches, they certainly are not present in the human subject, though I have found them existing in some of the niammalia, and most visibly in the dog and pig. They are striped muscles, the elements of which, the elongated cells or "fibre cells" (fig. 522.) which I have described*, are deposited in consider able quantity amongst the elastic network and white fibrous tissue previously mentioned.
In addition to these t two animals I have also • found th e m uscular structure in the cat, the ass, and the Dicotyles torquatus, while it was absent in the rabbit, horse, ox, hedgehog, guineapig, and bat. The elastic fibres of this tunic are for the most part much stronger than in man.